NFOv10

Hunt Hypothesis

This detection identifies the execution of files matching the specific NFOv10 signature pattern, which often indicates early-stage reconnaissance or the presence of known benign artifacts that could be repurposed by adversaries. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to validate these occurrences against current threat intelligence and distinguish between legitimate system activity and potential low-severity indicators of compromise before they escalate into critical incidents.

YARA Rule

rule NFOv10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8D 50 12 2B C9 B1 1E 8A 02 34 77 88 02 42 E2 F7 C8 8C }

condition:
		$a0 at pe.entry_point
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the NFOv10 detection rule in an enterprise environment, along with recommended filters and exclusions:

• Scenario: Microsoft Defender Antivirus Signature Updates

  • Context: The Windows Update service (wuauserv) or the Defender engine itself frequently downloads and unpacks new definition signatures. This process often involves creating temporary files that match the NFOv10 YARA signature due to specific header structures in the update payload.
  • Filter/Exclusion: Exclude file paths containing C:\ProgramData\Microsoft\Windows Defender\ and processes running under MsMpEng.exe. Additionally, filter out events where the parent process is svchost.exe with the service name wuauserv.

• Scenario: Scheduled Office 365 ProPlus Click-to-Run Updates

  • Context: The Office Click-to-Run update mechanism (OfficeClickToRun.exe) periodically scans and patches installed Office suites. During these checks, it generates temporary configuration files that trigger the NFOv10 rule due to similar binary structures found in the detection logic.
  • Filter/Exclusion: Create an exclusion for the process OfficeClickToRun.exe located within C:\Program Files\Microsoft Office Root\. Furthermore, exclude file extensions .msi, .cab, and .xml generated by this specific process during scheduled maintenance windows (e.g., 02:00 – 04:00 local time).

• Scenario: SCCM/Intune Application Deployment Packages

  • Context: When System Center Configuration Manager (SCCM) or Microsoft Intune pushes application deployments, the client agent (ccmsetup.exe or DeviceCensusAgent) extracts installation packages. These extraction processes often create intermediate files that mimic the signature patterns defined in N